For more than a century, it has been known that certain materials are sensitive to actinic radiation and, upon exposure to such radiation, form latent images capable of being subsequently developed into useful visible images. Almost exclusively, commercial application of these radiation sensitive materials has been the domain of silver halides which exhibit superior sensitivity to light over other radiation sensitive materials, some of which have been known for as long as silver halides have been in use. Such superior sensitivity has made silver halides more practical for use in cameras and other photographic equipment since they can be utilized in low light situations, or in situations where the mechanical characteristics of a camera (or other exposure means) would interfere with an optimum exposure.
Despite their superior sensitivity to light, there nevertheless has been considerable effort devoted to improving the sensitivity of silver halide crystals, and hence the photographic elements in which they are contained. In this regard, photographic chemists have attempted to vary the processes for making, or the components within, silver halide emulsions. One particularly preferred means by which to improve sensitivity has been to chemically sensitize photographic emulsions with one or more compounds containing labile atoms of gold, sulfur, selenium or the like. Examples of chemically sensitized photographic silver halide emulsion layers are described in, for example, Research Disclosure, Item No. 308119, December 1989, Section III, and the references listed therein. (Research Disclosure is published by Kenneth Mason Publications Ltd, Dudley Annex, 12a North Street, Emsworth, Hampshire PO 10 7DQ, England.)
Among the Au(I) chemical sensitizers known in the industry, trisodium aurous dithiosusulfate is often cited as being advantageous. This compound, however, is not universally applicable to all emulsion systems because of certain disadvantages it provides. In particular, trisodium aurous dithiosulfate contains two thiosulfate ions that are bonded to gold. These ions may undergo sulfur sensitization reactions in addition to the gold sensitization reactions in the emulsion. Therefore, this Au(I) compound is not appropriate in silver halide compositions in which a ratio of sulfur to gold of less than 2:1 is desired for chemical sensitization, and also not appropriate in silver halide compositions in which sulfur or selenium sensitizers other than thiosulfate are desired.
Other known Au(I) chemical sensitizers include aurous sulfides and the Au(I) thiolate compounds as described in U.S. Pat. No. 3,503,749. With respect to the former, although relatively easy to manufacture, they have been known to provide considerable sensitization variability and thus more predictable alternatives are desired. With respect to the latter compounds, they contain a sulfonic acid substituent on the thiolate ligand to impart water solubility. Further, they require the use of gold fulminate in their manufacture, a compound that is dangerously explosive and thus not desirable for practical use.
U.S. Pat. No. 5,049,485 describes a new class of Au(I) compounds comprising mesoionic ligands. Specifically, the Au(I) compounds contain one or two mesoionic substituents bound directly to a gold atom. The compounds are also positively charged, and thus must be associated with an appropriate anion, typically a halogen or tetrafluoroborate. The compounds described in U.S. Pat. No. 5,049,485 are advantageous in that they provide Au(I) sensitization without many of the disadvantages inherent in the use of the aforementioned Au(I) compounds. However, they have been known to exhibit limited stability in solution or dispersion. Further, at certain levels and under certain photographic conditions, they can cause an undesirable increase in fog.
Various other Au(I) compounds have been described. U.S. Pat. Nos. 5,252,455 and 5,391,727 disclose the use of bis Au(I) bidentate macrocyclic cationic sensitizers. U.S. Pat. No. 5,049,484 teaches the use of bis Au(I) sensitizers wherein the Au atom is ligated to the nitrogen atoms of two heterocyclic rings. U.S. Pat. No. 5,220,030 teaches the use of Au(I) compounds with bis mesoionic heterocycles. U.S. Pat. No. 5,620,841 discloses the use of gelatin dispersions of Au(I) thiosulfonato sensitizers with two different ligands comprising at least one ligand that is mesoionic. There is increasing evidence that the bis mesoionic sensitizers lack solution stability such that they require refrigeration to prevent decomposition. There are also difficulties in the preparation of the starting materials and the mesoionic ligand for such compounds. In general, the ligands of all of these bis Au(I) compounds are somewhat difficult to make. U.S. Pat. No. 5,700,631 teaches the use of gelatin dispersions of Au(I) thiosulfonato sensitizer with two different ligands comprising at least one ligand that is a thioether group.
There is still need for a Au (I) compound that can provide chemical sensitization without a concurrent and substantial rise in fog levels. These compounds should be stable in solution or dispersion and should be suitable for multiple types of emulsion systems. Further, they should be readily synthesizable in the absence of dangerous and unstable reactants.